The Substrate
The bitumen deposits in the Athabasca tar sands in Alberta, Canada are estimated to contain at least 1.7 trillion barrels of oil, and as such may represent around one-third of the world's total petroleum resources. Over 85% of known bitumen reserves lie in this deposit, and their high concentration makes them economically recoverable. Other significant deposits of tar sands exist in Venezuela and the USA, and similar deposits of oil shale are found in various locations around the world. These deposits consist of a mixture of clay or shale, sand, water and bitumen.
Bitumen is a viscous, tar-like material composed primarily of polycyclic aromatic hydrocarbons (PAHs). PAHs have a low hydrogen-to-carbon content and are difficult to extract and process. Extraction of the useful bitumen in tar sands is a non-trivial operation, and many processes have been developed or proposed. Lower viscosity deposits can be pumped out of the sand, but more viscous material is generally extracted with superheated steam, using processes known as cyclic steam stimulation (CSS) or steam assisted gravity drainage (SAGD). More recently, this latter technology has been adapted to use hydrocarbon solvents instead of steam, in a vapor extraction (VAPEX) process. Supercritical fluids (SCFs) have been considered a potentially attractive extractant for bituminous deposits since the 1970s. Their low densities and low viscosities make them particularly effective at permeating tar sands and oil shales and extracting organic deposits, and the energy costs associated with the moderate temperatures and pressures required to produce them compare very favourably with those processes that use superheated steam. For example, bitumen has been successfully recovered from Stuart oil shale in Queensland using supercritical carbon (sc) dioxide (scCO2), and from Utah oil sands using supercritical propane (sc propane). Very recently, Raytheon announced the use of scCO2 in combination with RF heating to extract oil shale deposits beneath Federal land in Colorado, Utah and Wyoming.
Bitumen typically contains around 83% carbon, 10% hydrogen and 5% sulfur by weight, along with significant ppm amounts of transition metals like vanadium and nickel associated with porphyrin residues. This low-grade material commonly needs to be converted into synthetic crude oil or refined directly into petroleum products before it can be used for most applications. Typically, this is carried out by catalytic cracking, which redistributes the hydrogen in the material. Catalytic cracking produces a range of ‘upgraded’ organic products with relatively high hydrogen content, but leaves behind a substance known as asphaltene, which is even more intractable than bitumen and contains very little hydrogen. Unless this asphaltene is upgraded by reaction with hydrogen, it is effectively a waste product.
Catalytic hydrogenation of organic molecules is of vital importance in the fine chemicals and petrochemicals industries. Solution phase reactions employing H2 as the hydrogen source are usually slow, on account of the low solubility of this gas in conventional organic solvents. In recent years, supercritical carbon dioxide (scCO2) has emerged as an attractive alternative to conventional solvents for several reasons. These include its low cost and toxicity, the abundance of CO2 in the atmosphere, and the modest temperature and pressure required to form a supercritical phase. In addition, the use of scCO2 in place of organic solvents is increasingly viewed as an environmentally attractive substitution. In contrast to a conventional solvent environment, H2 is completely miscible with scCO2, and supercritical CO2/H2 mixtures have been the subject of much interest as reaction media for several hydrogenation processes.
Polycyclic aromatic hydrocarbons (PAHs) occur widely in terrestrial and extraterrestrial environments. Their high aromatic stabilisation energy renders them a thermodynamically favourable product of a variety of chemical processes. Thus, they are major constituents of heavy oils and coal deposits, where they arise from degradation of natural products such as steroids and porphyrins. They also appear to be widely distributed in interstellar space, where they are believed to be responsible for the cosmic unidentified infrared emission bands. Their low H:C ratio and high molecular weights means that PAHs have to be upgraded through catalytic cracking and hydrogenation before they can be used as a feedstock for conventional chemical or petrochemical processes.
Catalytic hydrogenation of simple PAHs such as naphthalene and anthracene has been achieved using severe reaction conditions (>300° C.; 5 MPa H2). The high aromatic stabilisation of fused-ring systems such as these renders them challenging substrates to hydrogenate, leading to lower reaction rates (relative rates of hydrogenation compared to benzene: benzene to cyclohexane=1, phenanthrene to tetrahydroanthracene=0.7). There have been sporadic reports in the literature describing the hydrogenation of PAHs under milder conditions. Thus, Shirai and co-workers achieved conversion of naphthalene to decalin in scCO2 at 60° C. with a Rh/C catalyst and H2 (6 MPa), and Marshall et al. reported catalytic hydrogenation of a variety of PAHs (μmol scale) under mild conditions in the presence of supported Pd using hexane or scCO2 as a solvent. Metalloporphyrin catalysts have also been used to achieve partial hydrogenation of naphthalene, anthracene and phenanthrene. However, there remains significant scope for improvements in these methods through a systematic approach.
A number of problems in extracting, handling and upgrading bitumen have been observed.
There is a need for systems and methods that allow for efficient, cost-effective and rapid processing of bitumen.